The present invention relates generally to certain compounds having optical properties. More particularly, as an example, an embodiment of the invention provides a specific compound comprising RiLajAlkB16O48, where 2.0≦i≦3.6, 0.4≦j≦2.0, i and j sum to about four, k is about 12, and R is selected from an elemental group consisting of Y and Lu, for use with ultraviolet, visible, and infrared electromagnetic radiation. More specifically, another embodiment provides a compound comprising YiLajAlkB16O48, where 2.8≦i≦3.2, 0.8≦j≦1.2, i and j sum to about four, and k is about 12, for use with ultraviolet, visible, and infrared electromagnetic radiation. Merely by way of example, the compound is useful for electromagnetic radiation having wavelengths of 175-360 nm, but it would be recognized that the invention has a much broader range of applicability.
Nonlinear optical (NLO) materials are unusual in that they affect the properties of light. A well-known example is the polarization of light by certain materials, such as when materials rotate the polarization vectors of incident light. If the effect on the polarization vector by the incident light is linear, then light emitted by the material has the same frequency as the incident light. NLO materials affect the polarization vector of the incident light in a nonlinear manner. As a result, the frequency of the light emitted by a nonlinear optical material is affected, also described as frequency conversion/converters.
For example, when a beam of coherent light of a given frequency, such as produced by a laser, propagates through a properly oriented NLO crystal having non-zero components of the second order polarizability tensor, the crystal will generate light at a different frequency, thus extending the useful frequency range of the laser. Generation of this light can be ascribed to processes such as sum-frequency generation (SFG), difference-frequency generation (DFG) and optical parametric amplification (OPA). Devices using NLO crystals include, but are not limited to up and down frequency converters, optical parametric oscillators, optical rectifiers, and optical switches.
Frequency generation in NLO materials is an important effect. For example, two monochromatic electromagnetic waves with frequencies ω1 and ω2 propagating through a properly oriented NLO crystal can result in generation of light at a variety of frequencies. Mechanisms defining the frequency of light using these two separate frequencies are sum-frequency generation (SFG) and difference-frequency generation (DFG). SFG is a process where light of frequency ω3 is generated as the sum of the two incident frequencies, ω3=ω1+ω2. In other words, SFG is useful for converting long wavelength light to shorter wavelength light (e.g. near infrared to visible, or visible to ultraviolet). A special case of sum-frequency generation is second-harmonic generation (SHG) where ω3=2ω1, which is satisfied when the incident frequencies are equal, ω1=ω2. DFG is a process where light of frequency ω4 is generated as the difference of the incident frequencies ω4=ω1−ω2. DFG is useful for converting shorter wavelength light to longer wavelength light (e.g. visible to infrared). A special case of DFG is when ω1=ω2, hence ω4=0, which is known as optical rectification. Optical parametric oscillation (OPO) is also a form of DFG and is used to produce light at tunable frequencies.
The conversion efficiency of an NLO crystal for a particular application is dependent on a number of factors that include, but are not limited to: the effective nonlinearity of the crystal (pm/V), birefringence (Δn, where n is a refractive index), phase-matching conditions (Type I, Type II, non-critical, quasi, or critical), angular acceptance angle (radian-cm), temperature acceptance (K-cm), walk-off (radian), temperature dependent change in refractive index (dn/dT), optical transparency range (nm), optical damage threshold (W/cm2), and optical longevity. Desirable NLO crystals possess an optimal combination of the above properties as defined by specific applications.
Optical materials commonly use boron as an elemental constituent because of its wide transparency and its robust bonding in oxides. Examples include its use as glass-formers (borosilicate glasses), phosphors in the form of powders, and as laser frequency converters. Borate crystals are used in various applications, such as laser-based manufacturing, medicine, hardware and instrumentation, communications, and research studies. Several borate compounds are commonly used as crystals in commercial lasers: beta barium borate (BBO: β-BaB2O4), lithium triborate (LBO: LiB3O5), and cesium lithium borate (CLBO: CsLiB4O10). These crystals are examples of borate-based NLO crystals developed in recent years that are being used widely as NLO devices, especially in applications that use ultraviolet light. Select properties suitable for generation of laser light from the mid-infrared to the ultraviolet for these crystals are listed in Table 1.
TABLE 1Commercially Available NLO Materials and PropertiesPROPERTYBBOLBOCLBODeff (pm/V)2.20.80.9Optical Transmission (nm)190-3500160-2600180-2750Angular Acceptance (mrad-cm)0.86.50.6Temperature Acceptance (K-cm)557.52.5Walk-off Angle (deg.)30.61.8Damage Threshold (GW/cm2)51010Crystal Growth Propertiesflux or congr.fluxcongruent
BBO has a favorable nonlinearity (about 2.2 pm/V), transparency between 190 nm and 3500 nm, significant birefringence (necessary for phase matching), and a good damage threshold (5 GW/cm2, 1064 nm, 0.1 ns pulse width). However, its high birefringence creates a relatively small angular acceptance that can limit conversion efficiencies and laser beam quality. The crystal is somewhat hygroscopic and is limited on the amount of optical power that can be transmuted.
LBO exhibits optical transparency throughout the visible electromagnetic spectrum, extending well into the ultraviolet (absorption edge at about 160 nm), and possesses a high damage threshold (10 GW/cm2, 1064 nm, 0.1 ns pulse width). However, it has insufficient intrinsic birefringence for phase-matching to generate deep UV radiation.
CLBO appears capable of producing UV light due to a combination of high nonlinearity and sufficient birefringence. The crystal can also be manufactured to relatively large dimensions. However, the crystal is exceedingly moisture sensitive and invariably absorbs water from the air; hence, extreme care usually must be taken to manage environmental moisture to prevent hydration stresses and possible crystal destruction.
Frequency conversion generally benefits from both high peak powers and tightly focused input beams, both of which increase the intensity of the input and output beams within the nonlinear optical material. However, the lifetime of NLO materials under such conditions for UV production limits the usefulness of frequency-converted UV laser systems. Commercial DUV NLO devices of prior art are generally fabricated from BBO and CLBO crystals. These NLO devices are unable to support long term, high-output UV light because of their intrinsic weakness to moisture. Water interacts with the material's surfaces and penetrates into its bulk, causing breakdown in the presence of high intensity laser beams. Previous attempts to mitigate this failure mode included using environmental isolation with hermetic cells, elevated temperatures to reduce water sorption, purging dry gasses, and mechanical devices to shift the position of the crystal relative to the laser beam. Ultimately, it is very difficult to overcome the intrinsic material failings of BBO and CLBO for deep UV NLO processes.
A related consequence to the hygroscopic nature of BBO and CLBO is that these NLO materials are limited in the degree to which they are able to support high intensity radiation. With activation energy supplied by high intensity input beams, surface damage on the polished faces is quickly promoted in the presence of water. The degradation propagates along the beam path into the bulk device, driven by the high intensity laser beam. This phenomenon limits the amount and duration of input laser radiation through the frequency converter. As a result, conversion efficiencies remain well below optimum and device operational lifetimes are significantly compromised. Clearly, a new UV frequency converter that is impervious to water represents the real solution to the problem.
To address these concerns for conventional UV NLO crystals, several more recent materials have been considered but have not yet realized commercial relevance: compounds such as potassium aluminum borate (K2Al2B2O7), yttrium lanthanum scandium borate ((Y,La)Sc3(BO3)4), and strontium beryllium borate (Sr2Be2B2O7). These materials have appeared in research discussions and offered improved resistance against moisture intrusion, but issues such as crystal growth constraints, inadequate or unsuitable optical properties, difficulty of manufacture, laser damage, etc. have prevented these and other candidates from becoming practical crystals for frequency conversion.
Another material considered as a UV-grade NLO frequency converter is YAl3(BO3)4. This base formulation was put forward in 1960 by Ballman, and his potassium molybdate solvent of making crystals therein has remained as the primary means of growing crystal. As such and through the years, the pure form of YAB has not been commercially produced. The conventional method of production yields small crystal that contains a large amount of nonstoichiometric metals contamination and exhibits substandard crystal quality. Moreover, the solvent used introduces a considerable amount of contaminant that prevents device operation in the ultraviolet. The summary of work on huntite borates by Leonyuk & Leonyuk (1995) described a flux system that has subsequently remained as a method of producing YAB and its family members, namely the potassium molybdates K2MoO4 and K2Mo3O10. Unfortunately, these solvent formulations possess severe limitations for large scale crystal growth: a) high flux volatility, b) small crystal yield, and c) significant inclusion of Mo atoms into the crystalline structure. This latter issue revealed the lower spectral limit of optical use, described as 350-360 nm.
Again, its operation and the historic method of preparation limit its use to the visible and infrared. Hence, it is highly desirable to improve techniques for this family of compounds that enable optical function down into the ultraviolet. Thus, there is a need in the art for improved methods and techniques for optical compounds.